1. Field of the Invention
The present invention relates to a power transmission device, and more particularly to clutch device in a power transmission device of a hybrid vehicle which includes a fluid pressure generating mechanism that absorbs energy in the form of braking torque from the vehicle's tires during deceleration of the vehicle.
2. Description of the Related Art
Pressure accumulating hybrid vehicles are motorized vehicles that are configured to absorb energy in the form of braking torque from the vehicle's tires during deceleration of the vehicle. The absorbed energy is then used for starting motion of the vehicle again. The vehicle includes a hydraulic,pump/motor that is mechanically connected to the vehicle tires. In such a pressure accumulating hybrid vehicle, the hydraulic pump/motor is driven by braking torque from the vehicles tires during deceleration or braking The braking torque is directed to the hydraulic pump/motor which, during deceleration, serves as a hydraulic pump thereby accumulating fluid pressure Then, when starting motion of the motor vehicle, the energy accumulated from braking torque during deceleration is utilized to rotate the hydraulic pump/motor, which now serves as a hydraulic motor. The drive force of the hydraulic motor is utilized as an auxiliary drive force to rotate the tires.
In the pressure accumulating hybrid vehicle described above, the hydraulic pump/motor is mechanically connected to the tires of the vehicle at all times and therefore is driven during regular motion of the vehicle. Consequently, the hydraulic pump/motor creates an energy consuming load whenever the vehicle is in motion.
In order to selectively interrupt transmission of torque to the hydraulic pump/motor, a clutch device could be provided between the hydraulic pump/motor and the tires. In general, a solenoid clutch or the like would appear to be suitable for the clutch device. However, since the solenoid clutch is heavy in weight and is generally an expensive component, it is not suitable, since by their very nature such hybrid vehicles are meant to reduce costs. In particular, in case of a large size vehicle such as a bus, since the forces associated with braking torque may be extremely large, it is very difficult to inexpensively design a clutch device which may transmit a large amount of torque, and yet keep the weight of the clutch light in weight and remain inexpensive.
Another possible solution is a power switching device which may effect gear engagement and disengagement between wheels and the hydraulic pump/motor. Such a gear device might be utilized for transmission and interruption of torque transmission in place of a clutch device. The torque is transmitted directly by the engagement of gears and therefore device may be made relatively small in size. However, it is necessary to disengage the gears during the regular motion of the vehicle and engage the gears with each other when the braking is effected during traveling in order to decelerate. The system suffers in that a shock or jolt may be experienced during the engagement of the gears and it is difficult to smoothly perform the engagement of the gears.